Number in hibernation in effect (4)
I believe the answer is:
nine
'number' is the definition.
(nine is a number)
'in hibernation in effect' is the wordplay.
'in' indicates a hidden word.
'NINE' is hidden in the letters of 'hibernation in effect'.
(Other definitions for nine that I've seen before include "Number of lepers who failed to return" , "Three squared" , "'Possession is... points of the law'" , "A perfect square" , "Number for a club" .)